17.S 



di.scovi:hy 



ever, is so slight as to be imperceptible ; what actually 

 happens is a slight shift of the entire spectrum in one 

 direction or another. There is, therefore, a displace- 

 ment of the spectral lines in one direction if the source 

 of light is approaching, and in another direction if it is 

 receding. The application of Doppler's principle to 

 the measurement of the rotation and atmospheric 

 motions of the sun was one of the epoch-making 

 advances in this period. In the hands of Duner, the 

 Swedish astronomer, the spectroscope confirmed the 

 discovery by Carrington and Sporcr of the sun's 

 equatorial acceleration, and indicated that its rotation 

 period was protracted to about thirty-eight days near 

 the poles. Measures of the velocities of prominences 

 were also secured by Young and otliers. 



In 1891 the third period of solar research was inau- 

 gurated. It may be called the spectroheliographic 

 period, from the instrument which in that year became 

 the invaluable adjunct to telescope and spectroscope. 

 The spectroheliograph was devised independently by 

 Professor George E. Hale, now director of the Mount 

 Wilson Observatory, in California, who was then in 

 charge of a private observatory near Chicago, and 

 by M. Henri Deslandres, of Meudon, near Paris. The 

 principle of the spectroheliograph has been lucidly 

 explained by Professor R. A. Sampson, in his admirable 

 little monograph The Sun, in the following words : In a 

 spectrogram, or photograph of a spectrum, " each line is 

 a record of the presence and state of a separate chemical 

 element at the spot of the disc to which the slit is 

 directed. If this record could be read for that special 

 line for the whole disc, we should have the same infor- 

 mation summed up for the whole sun. . . . Let the 

 light from the line in question be allowed to pass 

 to the photographic plate, by means of a second slit, at 

 the focus of the camera, the jaws of which shut off all 

 the rest of the spectrum. Let both the first and the 

 second slits be long enough to extend right across the 

 image of the sun. Move the image of the sun across 

 the first slit, then the light which passes through the 

 second slit will come at every moment from different 

 strips of the sun's surface ; and if the photographic 

 plate be moved behind the second slit, in unison with 

 the movement of the sun's image across the first slit, 

 a record will be given, not of the radiations of every 

 substance mixed together, as in ordinary photographs 

 or visual observations of the sun's disc, but of the states 

 of some isolated substances as calcium or hydrogen, 

 and even of different strata of these." 



The early work both of Hale and Deslandres was 

 largely experimental. Both astronomers were handi- 

 capped by limited opportunities and instrumental 

 means. In 1908, however, on the assumption by Des- 

 landres of the post of director of the Meudon Observa- 

 tory, that institution was thoroughly rc-cquipped, and 



the director began the construction of a large spectro- 

 heliograph " to reveal the superior layers of the 

 chromosphere • by the aid of the light of calcium and 

 hydrogen." In addition to the spectroheliograph, 

 Deslandres devised an instrument which he called 

 the " spectro-cnregisteur des vitesses " — the velocity- 

 recorder. With this form of spectroheliograph it is 

 possible to determine, by means of Doppler's principle, 

 the radial velocities all over the disc, indicating which 

 vapours are ascending and which are descending. As a 

 result of the work of Hale, Deslandres, and other obser- 

 vers — notably those working under Hale at Mount Wil- 

 son — it is now possible to speak as Deslandres has done 

 of the " progressive revelation of the entire atmosphere 

 of the sun." The last twenty years have been a period 

 of unparalleled progress in solar astronomy. The new 

 methods, for instance, have thrown a flood of light 

 on the nature of the solar spots. These spots were 

 among the first-fruits of Galileo's little refractor, and 

 have been under telescopic observation for over three 

 centuries. It is indeed somewhat remarkable that, 

 although they were obser\'ed carefully for two hundred 

 years by astronomers of such consummate skill as 

 Cassini, Wilson, and the elder Herschel, their perio- 

 dicity was not discovered until Schwabe, with a small 

 hand telescope, began to count their numbers from day 

 to day. The telescope, unaided by its modern adjuncts, 

 however, was of little use for fathoming the mystery of 

 the constitution of the spots. For many years Wilson 's 

 classical theory of the nature of the spots was widely 

 accepted. This hypothesis, which was adopted by 

 Herschel, maintained that they represented depressions 

 below the surface of the solar photosphere. Careful 

 observation in the latter half of the last century, 

 however, led a number of competent investigators 

 first to doubt and latterly to challenge the accuracy of 

 the theory. In 1904 the late Professor C. A. Young, 

 of Princeton, cautiously pointed out that "as regards 

 the sun-spots it seems no longer safe to assume that they 

 are always depressions in the photosphere." More 

 definite was the conclusion reached by Dr. Abbott in 

 1911, that " it seems most probable that the level of 

 the sun-spot phenomena seen by ordinary obser\^ers 

 differs very little, if at all, from that of the surrounding 

 bright surface of the sun." 



By 1903, the development of the spectroheliograph 

 permitted the photography of hydrogen clouds as well 

 as calcium. In the spring of 1908 Hale, at Mount 

 \\'ilson, made the interesting experiment of photo- 

 graphing the sun in the red light of hydrogen. These 

 photographs revealed whirling storms in a region of the 

 solar atmosphere higher than that of the calcium and 



' The photosphere is the himinous envelope of the sun. The 

 chromosphere is the gaseous envelope of the sun through which 

 light passes from the photosphere. 



